Microbend-resistant optical fiber

ABSTRACT

A bend-insensitive glass fiber with a novel coating system yields exceptionally low losses. The coating system features (i) a softer primary coating with excellent low-temperature characteristics to protect against micro-bending in any environment and in the toughest physical situations and, optionally, (ii) a colored secondary coating possessing enhanced color strength and vividness. The secondary coating provides improved ribbon characteristics for structures that are robust, yet easily entered (i.e., separated and stripped). The optional dual coating is specifically balanced for superior heat stripping in fiber ribbons, with virtually no residue left behind on the glass. This facilitates fast splicing and terminations. The improved coating system provides optical fibers that offer significant advantages for deployment in most, if not all, fiber-to-the-premises (FTTx) systems.

CROSS-REFERENCE TO PRIORITY APPLICATIONS

This U.S. nonprovisional application hereby claims the benefit of U.S.patent application Ser. No. 60/986,737 for a Microbend-Resistant OpticalFiber (filed Nov. 9, 2007), U.S. patent application Ser. No. 61/041,484(filed Apr. 1, 2008) for a Microbend-Resistant Optical Fiber, and U.S.patent application Ser. No. 61/112,595 for a Microbend-Resistant OpticalFiber (filed Nov. 7, 2008), each of which is incorporated by referencein its entirety.

FIELD OF THE INVENTION

The present invention embraces optical fibers possessing an improvedcoating system that reduces stress-induced microbending. The presentinvention further embraces the deployment of such optical fibers invarious structures, such as buffer tubes and cables.

BACKGROUND OF THE INVENTION

Fiber to the premises/business/home (i.e., FTTx) provides broadband datatransfer technology to the individual end-user. FTTx installations,which are being increasingly deployed throughout the world, are makinguse of innovative, reduced-cost system designs to promote the spread ofthe technology. For example, fiber may be delivered in the last link byway of a microcable. Air-blown fibers provide another efficient modelfor delivering the link to the end-use terminus. There continues to beindustry-wide focus on modes of deployment that overcome economicobstacles that impede fiber-based broadband solutions for datatransmission to businesses and residences.

Cost-effectiveness is important, of course, for achieving successfulFTTx systems. Reduced size for cables, drops, and structures for blowingare often critical, too. Installation of conduits suitable fortraditional cable designs is often prohibitive in existinginfrastructure. Thus, existing small ducts or tight pathways have to beused for new fiber installations. Low-cost and reduced-size requirementsare driving in a direction that reduces protection for the opticalfibers (i.e., away from conventionally robust, more bulky cabledesigns).

Glass designs are now available that offer reduced sensitivity to smallbending radius (i.e., decreased added attenuation due to the phenomenonknown as macrobending). These include trench-assisted core design orvoid-assisted fibers. Glass designs with lower mode field diameter areless sensitive to macrobending effects, but are not compatible with theG.652 SMF standard. Single-mode optical fibers that are compliant withthe ITU-T G.652.D requirements are commercially available, for instance,from Drake Comteq (Claremont, N.C.).

Microbending is another phenomenon that induces added loss in fibersignal strength. Microbending is induced when small stresses are appliedalong the length of an optical fiber, perturbing the optical paththrough microscopically small deflections in the core.

In this regard, U.S. Pat. No. 7,272,289 (Bickham et al.), which ishereby incorporated by reference in its entirety, proposes an opticalfiber having low macrobend and microbend losses. U.S. Pat. No. 7,272,289broadly discloses an optical fiber possessing (i) a primary coatinghaving a Young's modulus of less than 1.0 MPa and a glass transitiontemperature of less than −25° C. and (ii) a secondary coating having aYoung's modulus of greater than 1,200 MPa.

Nonetheless, better protection against microbending is still needed tohelp ensure successful deployment in more FTTx applications. To thisend, it is necessary to discover and implement new coating systems thatbetter address the demands FTTx installations place on fiber and cablestructures in a way that is commercially practical (i.e.,cost-effective).

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide anoptical fiber having an improved coating system that provides improvedprotection against stress-induced microbending.

It is another object to provide an improved coating system that can bereadily mated with bend-insensitive optical fiber, as well asG.652-compliant fiber.

It is yet another object to provide an improved optical fiber coatingsystem including a primary coating that possesses a low modulus toprovide enhanced cushioning against lateral and axial stresses inducedby external forces.

It is yet another object to provide an improved optical fiber coatingsystem including a primary coating that possesses an exceptionally lowglass transition temperature (T_(g)) that reduces temperature-inducedstresses in unusually cold environments.

It is yet another object to provide an improved optical fiber coatingsystem including a primary coating that possesses an improved curingrate.

It is yet another object to provide an improved optical fiber coatingsystem including an ink-free secondary coating that has improvedbrightness and visibility.

It is yet another object to provide an improved optical fiber coatingsystem that can be applied at commercial processing speeds (e.g.,forming the primary coating at rates of at least about 20 meters persecond).

It is yet another object to provide an optical fiber possessing coatingsthat are readily stripped.

It is yet another object to provide an optical fiber having enhancedperformance characteristics for use in FTTx installations in whichconventional, robust cable designs are impractical.

It is yet another object to provide an optical fiber thatsynergistically combines a bend-insensitive glass fiber (e.g., DrakaComteq's single-mode glass fibers available under the trade nameBendBright^(XS)®) with the coating according to the present invention(e.g., Draka Comteq's ColorLock^(XS) brand coating system).

It is yet another object to provide an optical fiber that can beadvantageously deployed in buffer tubes and/or fiber optic cables.

It is yet another object to provide an optical fiber that requires lessexternal protection (e.g., enclosed within thinner buffer tubes and/orcable jacketing).

It is yet another object to provide a bend-insensitive optical fiberpossessing a reduced diameter (e.g., having thinner coating layers).

It is yet another object to provide an optical fiber that can beinstalled in a way that employs small-radius bends.

It is yet another object to provide an optical fiber that facilitatesdirect installation onto buildings or other structures (e.g., stapled orotherwise secured to structural surfaces).

The foregoing, as well as other objectives and advantages of theinvention, and the manner in which the same are accomplished, arefurther specified within the following detailed description and itsaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts microbend testing results demonstrating thatexceptionally low microbending losses are achieved, in accordance withthe present invention, by pairing a bend-insensitive glass fiber with alow-modulus primary coating.

FIG. 2 schematically depicts the relationship between the in situmodulus of a primary coating and added loss for a multimode opticalfiber.

FIG. 3 depicts the dynamic mechanical properties of a typical commercialprimary coating (i.e., a conventional primary coating).

FIG. 4 depicts the dynamic mechanical properties of an exemplary primarycoating used in producing optical fibers according to the presentinvention.

FIG. 5 depicts microbend testing results for optical fibers that includea conventional primary coating and for optical fibers that include anexemplary primary coating according to the present invention.

FIG. 6 depicts microbend testing results (under rigoroustemperature-cycle testing conditions) for optical fibers that include aconventional primary coating and for optical fibers that include anexemplary primary coating according to the present invention.

FIG. 7 depicts microbend testing results (under modifiedtemperature-cycle testing conditions) for optical fibers that include aconventional primary coating and for optical fibers that include anexemplary primary coating according to the present invention.

FIG. 8 depicts microbend testing results demonstrating thatexceptionally low microbending losses are achieved, in accordance withthe present invention, by pairing a bend-insensitive glass fiber with alow-modulus primary coating.

FIG. 9 depicts microbend testing results (under rigoroustemperature-cycle testing conditions) for conventional optical fibersand for optical fibers that, in accordance with the present invention,combine a bend-insensitive glass fiber with a low-modulus primarycoating.

FIG. 10 depicts microbend testing results (under modifiedtemperature-cycle testing conditions) for conventional optical fibersand for optical fibers that, in accordance with the present invention,combine a bend-insensitive glass fiber with a low-modulus primarycoating.

FIG. 11 depicts attenuation (added loss) as a function of MAC number(i.e., mode field diameter divided by cutoff wavelength) for variousexemplary optical fibers.

FIG. 12 depicts, on a logarithmic scale, microbend sensitivity as afunction of MAC number (i.e., mode field diameter divided by cutoffwavelength) for various exemplary optical fibers.

DETAILED DESCRIPTION

In one aspect, the present invention embraces optical fibers possessingan improved coating system that reduces stress-induced microbending,even in exceptionally cold environments required for FTTx deployments.The coating system according to the present invention includes a primarycoating that combines low in situ modulus (e.g., less than about 0.5 MPaas measured on the fiber) and low glass transition temperature (T_(g))(e.g., less than about −50° C.) to reduce stresses caused by externalforce and temperature. In addition, the coating system can be processedat high production speeds (e.g., 15-20 m/sec or more).

The present invention achieves a microbend-resistant optical fiber,particularly a single-mode optical fiber, by employing as its primarycoating a UV-curable, urethane acrylate composition. In this regard, theprimary coating includes between about 40 and 80 weight percent ofpolyether-urethane acrylate oligomer as well as photoinitiator, such asLUCIRIN® TPO, which is commercially available from BASF. In addition,the primary coating includes one or more oligomers and one or moremonomer diluents (e.g., isobornyl acrylate), which may be included, forinstance, to reduce viscosity and thereby promote processing. A suitablecomposition for the primary coating according to the present inventionis a UV-curable urethane acrylate product provided by DSM Desotech(Elgin, Illinois) under the trade name DeSolite® DP 1011.

In this regard, this application incorporates entirely by reference thefollowing commonly assigned patent applications: U.S. Patent ApplicationNo. 60/986,737 for a Microbend-Resistant Optical Fiber (Overton), U.S.Patent Application No. 61/041,484 (Overton) for a Microbend-ResistantOptical Fiber, U.S. Patent Application No. 61/112,595 for aMicrobend-Resistant Optical Fiber (Overton), and International PatentApplication No. PCT/U.S.08/82927 Microbend-Resistant Optical Fiber(Overton).

Suitable glass fibers for use in the present invention include glassfibers such as those disclosed in U.S. Pat. No. 4,838,643 for a SingleMode Bend Insensitive Fiber for Use in Fiber Optic Guidance Applications(Hodges et al.); U.S. Patent Application Publication No. US 2007/0127878Al and its related U.S. patent application Ser. No. 11/556,895 for aSingle Mode Optical Fiber (de Montmorillon et al.); U.S. PatentApplication Publication No. US 2007/0280615 Al and its related U.S.patent application Ser. No. 11/697,994 for a Single-Mode Optical Fiber(de Montmorillon et al.); U.S. Pat. No. 7,356,234 and its related U.S.patent application Ser. No. 11/743,365 for Chromatic DispersionCompensating Fiber (de Montmorillon et al.); U.S. Patent ApplicationPublication No. US 2008/0152288 Al and its related U.S. patentapplication Ser. No. 11/999,333 for an Optical Fiber (Flammer et al.);U.S. patent application Ser. No. 61/101,337 for a Single Mode OpticalFiber (de Montmorillon et al.); U.S. patent application Ser. No.61/112,006 for a Bend-Insensitive Single-Mode Optical Fiber (deMontmorillon et al.); and U.S. patent application Ser. No. 61/112,374for a Bend-Insensitive Single Mode Optical Fiber (de Montmorillon etal.). Each of these commonly assigned patent documents is herebyincorporated by reference in its entirety. One exemplary glass fiber,for instance, possesses a step-index core having a refractive index thatis between about 0.003 and 0.006 higher than the refractive index of itsadjacent silica cladding.

Exemplary single-mode glass fibers for use in the present invention arecommercially available from Draka Comteq (Claremont, N.C.) under thetrade name BendBright®, which is compliant with the ITU-T G.652.Drequirements, and the trade name BendBright^(XS)®, which is compliantwith the ITU-T G.657.A/B and ITU-T G.652.D requirements.

In particular and as set forth herein, it has been unexpectedlydiscovered that the pairing of a bend-insensitive glass fiber (e.g.,Draka Comteq's single-mode glass fibers available under the trade nameBendBright^(XS)®) and a primary coating having very low modulus (e.g.,DSM Desotech's UV-curable urethane acrylate product provided under thetrade name DeSolite® DP 1011) achieves optical fibers havingexceptionally low losses (e.g., reductions in microbend sensitivity ofat least 10× (e.g., 40× to 100× or more) as compared with a single-modefiber employing a conventional coating system). Draka Comteq'sbend-resistant, single-mode glass fiber available under the trade nameBendBright^(XS)® employs a trench-assisted design that reducesmicrobending losses.

FIG. 1 depicts this outstanding result by comparing the aforementionedexemplary single-mode fiber according to the present invention withvarious single-mode fibers employing conventional coating systems. Inthis regard, FIG. 1 presents spectral attenuation data by measuringinitial spectral attenuation on the optical fiber on a shipping spool,thereby obtaining the peaks and valleys typical of the attenuationacross the full spectrum of wavelengths between the limits shown. Theoptical fiber is then wound onto a sandpaper-covered, fixed-diameterdrum (i.e., measurement spool) as described by the IEC fixed-diametersandpaper drum test (i.e., IEC TR62221, Method B), and another spectralattenuation curve is obtained.

The IEC fixed-diameter sandpaper drum test (i.e., IEC TR62221, Method B)provides a microbending stress situation that affects single-mode fiberseven at room temperature. The sandpaper, of course, provides a roughsurface that subjects the optical fiber to thousands, if not millions,of stress points. With respect to the test data presented in FIG. 1, a300-mm diameter fiber spool was wrapped with adhesive-backed, 40-microngrade sandpaper (i.e., approximately equivalent to 300-grit sandpaper)to create a rough surface. Then, 400-meter fiber samples were wound atabout 2,940 mN (i.e., a tension of 300 gf on a 300-mm diametercylinder), and spectral attenuation was measured at 23° C.

The curves presented in FIG. 1 represent the difference between theinitial spectral curve and the curve when the fiber is on the sandpaperdrum, thereby providing the added loss due to microbending stresses.

* * *

Those having ordinary skill in the art will recognize cable designs arenow employing smaller diameter buffer tubes and less expensive materialsin an effort to reduce costs. Consequently, when deployed in such cabledesigns, single-mode optical fibers are less protected and thus moresusceptible to stress-induced microbending. As noted, the presentinvention provides an improved coating system that better protectsoptical fibers against stresses caused by external mechanicaldeformations and by temperature-induced, mechanical property changes tothe coatings.

As noted, conventional solutions for protecting optical fibers involvedusing large-diameter buffer tubes, buffer tubes made of high-modulusmaterials that resist deformation and stresses upon the fiber, andstronger, thicker cable jackets to resist deformations that might pinchor otherwise squeeze the optical fibers. These solutions, however, arenot only costly, but also fail to address the temperature-inducedstresses caused by changes to the protective coatings. In other words,conventional primary coatings possess high modulus at temperatures belowtheir respective glass transition temperatures.

As disclosed herein, the optical fiber according to the presentinvention includes a primary coating possessing lower modulus and lowerglass transition temperature than possessed by conventional single-modefiber primary coatings. Even so, the improved primary coatingformulation nonetheless facilitates commercial production of the presentoptical fiber at excellent processing speeds (e.g., 1,000 m/min ormore). In this regard, the primary coating employed in the opticalfibers of the present invention possesses fast curing rates—reaching 50percent of full cure at a UV dose of about 0.3 J/cm², 80 percent of fullcure at a UV dose of about 0.5 J/cm², and 90 percent of full cure at aUV dose of about 1.0 J/cm² as measured on a standard 75-micron film at20° C. and atmospheric pressure (i.e., 760 torr) (i.e., standardtemperature and pressure—STP).

FIG. 2 schematically depicts the observed relationship between the insitu modulus of a primary coating and the attenuation (added loss) ofthe optical fiber, here a 50-micron graded-index multimode fiber. Theprimary coating modulus is measured as cured on the glass fiber and theadded loss is measured using a fixed-diameter sandpaper drum procedurein accordance with the IEC TR62221 microbending-sensitivity technicalreport and standard test procedures (e.g., IEC TR62221, Method B, Ed.1), which are hereby incorporated by reference in their entirety.

As will be appreciated by those having ordinary skill in the art, prior,commercially available single-mode fibers typically include a Young'smodulus of 100-150 psi measured in situ (i.e., on the fiber). Theoptical fiber according to the present invention possesses a primarycoating having reduced modulus as compared with such commerciallyavailable primary coatings. Employing a lower modulus primary coatingprovides better cushioning around the glass fiber.

Although lower modulus of the in situ primary coating can be achieved byselectively undercuring, the present invention achieves in situ primarycoating having lower modulus even approaching full cure (i.e., near fullcure). In this regard, the modulus of the in situ primary coatingaccording to the present invention is less than about 0.65 MPa (e.g.,less than about 95 psi), typically less than about 0.5 MPa, and moretypically less than 0.4 MPa (e.g., between about 0.3 MPa and 0.4 MPa orbetween about 40 psi and 60 psi). It has been determined that an in situprimary coating having a modulus of less than about 0.5 MPasignificantly reduces bend sensitivity of the glass fiber. On the otherhand, the modulus of the in situ primary coating according to thepresent invention is typically greater than about 0.2 MPa (e.g., 0.25MPa or more).

To achieve its reduced modulus as compared with conventional opticalfiber coatings, the present primary coating possesses a lower crosslinkdensity, specifically a reduced concentration of the reactive acrylategroups. Those having ordinary skill in the art will appreciate thatacrylate groups crosslink via free radical polymerization duringphotoinitiation (e.g., UV-induced curing during drawing operations). Thereaction kinetics dictate reduced cure rates during processing. This iscommercially undesirable, of course, and so the present inventionimplements processing modifications to provide satisfactory cure ratefor the low-modulus primary coating.

There are at least two components of the curing process that retard therate of polymerization of the primary coating. First, the combination of(i) high curing temperatures induced by exposure to a high-intensity, UVenvironment and (ii) the exothermic polymerization reaction slows theobserved curing rate of the primary coating. Second, close proximity ofstacked UV lamps, in effect, creates rapidly superposed, repeatedphotoinitiation periods. The reaction rate of acrylate groups under thisconfiguration is likewise retarded—a somewhat counterintuitive result.With respect to the latter, disposing (i.e., positioning) UV lamps toincrease the period between consecutive UV exposures significantlyincreases the degree of coating cure as compared with other conventionalprocesses employing the same draw speed and UV dose. In this way, it ispossible to process the reduced-modulus, primary coating according tothe present invention in a way that achieves near-complete curing atfast fiber draw speeds, which are required for a commercially viableprocess. An exemplary method and apparatus for curing a coated fiber isdisclosed in commonly assigned U.S. Pat. No. 7,322,122, which is herebyincorporated by reference in its entirety.

The temperature dependence of the modulus is an important considerationto ensure that the primary coating provides enhanced microbendingprotection in FTTx applications. A primary coating having low modulusonly at room temperature would be inadequate because deployment in thefield will expose the optical fiber to microbend-inducing stresses atextreme environmental temperatures (e.g., −40° C. and below). Therefore,a suitable primary coating according to the present invention possessesan exceptionally low glass transition temperature so that the primarycoating remains soft and protective in extremely cold environmentalconditions.

Example 1 Comparison of Mechanical Properties

FIGS. 3 and 4, respectively, depict dynamic mechanical properties of atypical commercial primary coating (i.e., the conventional primarycoating) and an exemplary primary coating used in making the opticalfibers according to the present invention. The conventional primarycoating was a UV-curable urethane acrylate provided by DSM Desotech(Elgin, Ill.) under the trade name DeSolite® DP1007. The exemplaryprimary coating according to the present invention (i.e., employed toform optical fibers of the present invention) was a UV-curable urethaneacrylate provided by DSM Desotech (Elgin, Ill.) under the trade nameDeSolite® DP 1011.

The data for the conventional primary coating were obtained on a DynamicMechanical Analyzer (DMA) at an oscillatory stress rate of 1 Hz. Indoing so, the strain was maintained within the linear region ofstress-strain behavior. The sample of conventional primary coating wascured on polyester to form a standard 75-micron film. A UV dose of 1J/cm² was applied using a mercury-halide bulb operating at a 300 W/inoutput. This UV exposure was sufficient to ensure that the coating wason the plateau of the dose-modulus curve.

Referring to FIG. 3, the data show the equilibrium modulus to beapproximately 1.5 MPa as measured on a 75-micron film. On a glass fiber(i.e., in situ), this conventional primary coating typically cures wellto a modulus of about 0.8 MPa, a level indicative of many single-modefiber primary coatings in the industry. Those having ordinary skill inthe art will appreciate that modulus measurements of softer primarycoatings tend to be lower on a glass fiber (i.e., in situ) as comparedwith on a 75-micron film.

The glass transition temperature of the conventional primary coating isestimated by the peak in tan δ to be approximately −30° C. Thus, theconventional primary coating (and similar formulations) will behave likea glassy polymer at extremely low temperatures (e.g., less than −40° C.,particularly less than −50° C.). (Although stress induced by strain istime dependent at low temperatures, estimated glass transitiontemperature is a useful comparative property.)

A sample of the exemplary primary coating according to the presentinvention was likewise cured on polyester to form a comparable 75-micronfilm. As before, a UV dose of 1 J/cm² was applied to the primary coatingusing a mercury-halide bulb operating at a 300 W/in output. As noted,FIG. 4 depicts dynamic mechanical properties of the exemplary primarycoating according to the present invention.

The exemplary primary coating according to the present inventionexhibited an equilibrium modulus at just under 1 MPa in the cured film.The in situ modulus (i.e., measured on the glass fiber), was betweenabout 0.3 MPa and 0.4 MPa. This is significantly lower than therespective modulus measurements for the conventional primary coating.

The glass transition temperature of the exemplary primary coatingaccording to the present invention is estimated by the peak in tan δ atless than about −50° C. (e.g., about −60° C.). This is at least about20° C. below the glass transition temperature of the comparative,conventional primary coating. Accordingly, primary coatings according tothe present invention provide much more rapid stress relaxation duringtemperature excursions.

* * *

As set forth in Examples 2 and 3 (below), two different methods wereused to evaluate the respective microbend sensitivities of glass fiberscoated with (i) a typical commercial primary coating (i.e., theconventional primary coating) and (ii) an exemplary primary coatingaccording to the present invention. As with Example 1 (above), theconventional primary coating was a UV-curable urethane acrylate providedby DSM Desotech (Elgin, Ill.) under the trade name DeSolite® DP 1007,and the exemplary primary coating according to the present invention(i.e., employed to form optical fibers of the present invention) was aUV-curable urethane acrylate provided by DSM Desotech (Elgin, Ill.)under the trade name DeSolite® DP 1011.

Each test method provided aggravated lateral stress conditions.Moreover, after measuring the effect on attenuation at room temperature,the test structures were temperature cycled to determine the additionalloss induced by such temperature excursions.

Example 2 Comparison of Microbending Sensitivity

The first test method employed was a basket-weave, temperature cyclingprocedure known by those having ordinary skill in the art. According tothis test procedure, optical fiber was wound at about 490 mN (i.e., atension of 50 gf on a 300-mm diameter quartz cylinder with a 9-mm“lay”). Fifty layers were wound on the quartz drum to create numerousfiber-to-fiber crossovers. The testing procedure for Example 2 was anadaptation of IEC TR62221, Method D, which, as noted, is incorporated byreference in its entirety.

Those having ordinary skill in the art will appreciate that, at roomtemperature, such fiber crossovers can sometimes cause added loss (i.e.,if the optical fiber is very sensitive) but that typically little or noadded loss is observed. Consequently, the drum (with wound fiber) wastemperature cycled twice from about room temperature through (i) −40°C., (ii) −60° C., (iii) +70° C., and (iv) +23° C. (i.e., near roomtemperature) while making loss measurements at 1550 nanometers. In bothtemperature cycles, fiber attenuation was measured after one hour ateach test temperature.

FIG. 5 depicts exemplary results for single-mode glass fibers coatedwith, respectively, a conventional primary coating (i.e., DeSolite®DP1007) and an exemplary primary coating according to the presentinvention (i.e., DeSolite® DP 1011). The respective fiber specimens werechosen to match the coating geometry, mode field diameter, and cutoffwavelength. Accordingly, the respective optical fibers employeddifferent formulations of colored secondary coatings.

In summary, the conventional primary coating and the exemplary primarycoating according to the present invention each provided good protectionagainst microbending stresses at 23° C. Moreover, at −40° C., theoptical fiber having the conventional primary coating demonstrated onlya small added loss. (It would appear that at −40° C., the conventionalprimary coating provided adequate protection against microbending bystress relaxing in a reasonable timeframe, even though this was near itsglass transition temperature.) By way of comparison, the optical fiberaccording to the present invention demonstrated essentially no addedloss at −40° C. (i.e., better performance).

At −60° C., however, the optical fiber having the conventional primarycoating demonstrated significant added loss. (This temperature extremewas well below the glass transition temperature of the conventionalprimary coating.) By way of comparison, the optical fiber according tothe present invention demonstrated essentially no added loss at −60° C.,which is close to the glass transition temperature of this embodiment ofthe primary coating according to the present invention.

Example 3 Comparison of Microbending Sensitivity

The second test method employed more aggressive environments (i.e.,conditions) in order to evaluate the respective microbend sensitivitiesof (i) an optical fiber possessing a typical commercial primary coating(i.e., the conventional primary coating) and (ii) an optical fiberpossessing an exemplary primary coating according to the presentinvention.

In particular, the second method modified the IEC fixed-diametersandpaper drum test (i.e., IEC TR62221, Method B), which, as noted, isincorporated by reference in its entirety, to provide a microbendingstress situation sufficiently harsh to affect single-mode fibers even atroom temperature (i.e., a rougher drum surface than that used to measurethe data depicted in FIG. 1). To do this, a 300-mm diameter quartz drumwas wrapped with adhesive-backed, 220-grit sandpaper (i.e.,approximately equivalent to 66-micron-grade sandpaper) to create a roughsurface.

In an initial test condition, each of the respective fiber samples waswound in a single layer at about 980 mN (i.e., a tension of 100 gf on a300-mm diameter quartz cylinder). In a modified test condition, three(3) each of the respective fiber samples was wound in a single layer atabout 1,470 mN (i.e., a tension of 150 gf on a 300-mm diameter quartzcylinder). Thus, as compared with the first test condition, the secondtest condition increased the winding tension by 50 percent.

Using matched fiber samples (as with the basket weave/temperaturecycling test of Example 2) fiber attenuation was measured after windingat room temperature (i.e., 23° C.) for each test condition. Then, thedrum (with 400 meters of wound fiber) was temperature cycled from aboutroom temperature through (i) −40° C., (ii) −60° C., and (iii) +23° C.(i.e., near room temperature) while making loss measurements at 1550nanometers using an optical time domain reflectometer (OTDR).

The several samples of each kind of optical fiber were initiallymeasured at 23° C. on the original spools (i.e., before winding on theroughened drum surface to establish baseline spectral attenuation) thenwere subjected to the foregoing rigorous testing conditions for one hourat each temperature. Fiber attenuation was measured after one hour (asin Example 2) at each test temperature.

FIG. 6, a line chart, and FIG. 7, a box plot, depict exemplary resultsunder these more rigorous testing conditions for single-mode opticalfibers that include a conventional primary coating (i.e., DeSolite®DP1007 UV-curable urethane acrylate) and for single-mode optical fibersthat include an exemplary primary coating according to the presentinvention (i.e., DeSolite® DP1011 UV-curable urethane acrylate).

FIG. 6, for instance, shows that, as compared with conventional opticalfibers, exemplary optical fibers according to the present inventionpossess reduced microbend sensitivity (i.e., a reduction of about 40-60percent).

Likewise, FIG. 7 shows that, as compared with conventional opticalfibers, exemplary optical fibers according to the present inventionpossess substantially reduced microbend sensitivity at a higher windingtension (i.e., 150 gf on a 300-mm diameter quartz cylinder). FIG. 7 thusillustrates that the exemplary primary coating according to the presentinvention (i.e., DeSolite® DP1011 UV-curable urethane acrylate) promotesboth significantly reduced and significantly more uniform microbendingperformance.

In accordance with the foregoing, it has been found that, as comparedwith a conventional coating system, the present coating system providessignificant microbending improvement when used in combination with aconventional single-mode glass fiber.

It has been further found that pairing a bend-insensitive glass fiber(e.g., Draka Comteq's single-mode glass fibers available under the tradename BendBright^(XS)®) and a primary coating having very low modulus(e.g., DSM Desotech's UV-curable urethane acrylate product providedunder the trade name DeSolite® DP 1011) achieves optical fibers havingexceptionally low losses. Additional testing was performed, therefore,to demonstrate the dramatic and unexpected reductions in microbendsensitivity provided in accordance with the present invention.

Example 4 Comparison of Microbending Sensitivity

The respective microbend sensitivities were measured for exemplaryoptical fibers, including (i) a conventional single-mode glass fiberwith a conventional commercial coating, (ii) a bend-insensitive glassfiber with a conventional commercial coating, and (iii) abend-insensitive glass fiber (e.g., Draka Comteq's single-mode glassfibers available under the trade name BendBright^(XS)®) with the coatingaccording to the present invention (e.g., Draka Comteq's ColorLock^(XS)brand coating system).

FIG. 8 demonstrates that the optical fiber according to the presentinvention, namely including a bend-insensitive glass fiber (e.g., DrakaComteq's single-mode glass fibers available under the trade nameBendBright^(XS)®) and a primary coating having very low modulus (e.g.,DSM Desotech's UV-curable urethane acrylate product provided under thetrade name DeSolite® DP 1011), provides exceptionally low attenuationlosses as compared with other optical fibers. Moreover, thisbend-resistant optical fiber exhibits small wavelength dependence withinthe transmission window between 1400 nanometers and 1700 nanometers, andis essentially unaffected by the microbend-inducing test conditionsacross the test spectrum.

FIG. 8 presents exemplary spectral attenuation data obtained adhering toIEC TR62221, Method B (fixed-diameter drum). In accordance with IECTR62221, Method B, initial spectral attenuation was measured on a440-meter sample of optical fiber wound on a shipping spool (i.e.,obtaining the peaks and valleys typical of the attenuation across thefull spectrum of wavelengths between the limits shown). The opticalfiber was then wound at about 3 N onto a 300-mm diameter measurementspool wrapped with adhesive-backed, 40-micron grade sandpaper (i.e.,approximately equivalent to 300-grit sandpaper), and another spectralattenuation curve was obtained.

Like the curves presented in FIG. 1, the curves depicted in FIG. 8represent, at 23° C., the difference between the initial spectral curveand the curve when the fiber is on the sandpaper drum of fixed diameter,thereby providing the added loss due to microbending stresses (i.e.,delta-attenuation across the spectral range).

Example 5 Comparison of Microbending Sensitivity

The respective microbend sensitivities were measured under rigorous testconditions for exemplary optical fibers, including (i) a conventionalsingle-mode glass fiber with a conventional commercial coating and (ii)a bend-insensitive glass fiber (e.g., Draka Comteq's single-mode glassfibers available under the trade name BendBright^(XS)®) with the coatingaccording to the present invention (e.g., Draka Comteq's ColorLock^(XS)brand coating system).

FIG. 9 demonstrates that, even under extremely harsh conditions, theoptical fiber according to the present invention, namely including abend-insensitive glass fiber (e.g., Draka Comteq's single-mode glassfibers available under the trade name BendBright^(XS)®) and a primarycoating having very low modulus (e.g., DSM Desotech's UV-curableurethane acrylate product provided under the trade name DeSolite® DP1011), provides surprisingly low attenuation losses as compared withother optical fibers.

The testing procedure for Example 5 was an adaptation of IEC TR62221,Method B, which, as noted, is incorporated by reference in its entirety.For this modified IEC fixed-diameter sandpaper drum test, a 300-mmdiameter quartz drum was wrapped with adhesive-backed, 180-gritsandpaper (i.e., approximately equivalent to 78-micron-grade sandpaper)to create an even rougher surface than that described in Example 3(above). Then, 440-meter fiber samples were wound in a single layer atabout 1,470 mN (i.e., a controlled back tension of 150 gf on the 300-mmdiameter quartz cylinder using a Delachaux optical fiber windingapparatus), and spectral attenuation was measured.

FIG. 9 presents exemplary temperature-cycle data for three specimens ofstandard single-mode fiber (i.e., a conventional single-mode glass fiberwith a conventional commercial coating) and three specimens of opticalfiber according to the present invention (i.e., a bend-insensitive glassfiber with improved coating according to the present invention). Asnoted, 440 meters of optical fiber is wound onto the aforementionedsandpaper-covered, fixed-diameter drum. One hour after winding, fiberattenuation was measured at room temperature (i.e., 23° C.) using anoptical time domain reflectometer (OTDR). Then, the drum (with 440meters of wound fiber) was temperature cycled from about roomtemperature through (i) −40° C. and (ii) −60° C. in atemperature-controlled chamber. Fiber attenuation at 1550 nanometers wasmeasured by an OTDR after one hour of equilibration at both −40° C. and−60° C.

Microbending sensitivity (S_(m)) may be described as αR/T, wherein α isthe attenuation increase on the drum (dB/km), R is the radius of thefixed drum (mm), and T is the winding tension applied to the fiber (N).See e.g., IEC TR62221 Technical Report (Microbending Sensitivity). Inaddition to the parameters α, R, and T, however, themicrobending-sensitivity metric obtained from the fixed-diametersandpaper drum test is dependent on the coarseness of the sandpaperemployed on the measurement drum.

Table 1 (below) presents the microbending-sensitivity metric obtainedfrom the attenuation data (at a wavelength of 1550 nanometers) depictedin FIG. 9 (i.e., employing 180-grit sandpaper). Table 1 shows that, ascompared with a conventional standard single-mode fiber, the opticalfiber according to the present invention provides microbendingsensitivity that is about 2×-10× lower at 23° C. and about 2×-5× lowerat −40° C.:

TABLE 1 (Microbend Sensitivity) 23° C. −40° C. −60° C. Optical Fiber(dB/km)/ (dB/km)/ (dB/km)/ (Coating Color) (N/mm) (N/mm) (N/mm)Conventional SMF 139.9 220.6 331.8 (blue) Conventional SMF 261.0 329.7417.9 (red) Conventional SMF 104.3 161.9 228.0 (aqua) BendBright^(xs )®w/ 35.8 76.5 163.4 ColorLock^(xs) (slate) BendBright^(xs )® w/ 30.1 70.6144.2 ColorLock^(xs) (red) BendBright^(xs )® w/ 42.7 84.7 166.4ColorLock^(xs) (aqua)

Example 6 Comparison of Microbending Sensitivity

The respective microbend sensitivities were further measured forexemplary optical fibers, including (i) a conventional single-mode glassfiber with a conventional commercial coating and (ii) a bend-insensitiveglass fiber (e.g., Draka Comteq's single-mode glass fibers availableunder the trade name BendBright^(XS)®) with the coating according to thepresent invention (e.g., Draka Comteq's ColorLock^(XS) brand coatingsystem).

The testing procedure for Example 6 was an adaptation of IEC TR62221,Method B, which, as noted, is incorporated by reference in its entirety.For this modified IEC fixed-diameter sandpaper drum test, a 300-mmdiameter quartz drum was wrapped with adhesive-backed, 220-gritsandpaper (i.e., approximately equivalent to 66-micron-grade sandpaper)to create a rough surface like that described in Example 3. Each of thefiber samples was wound in a single layer at about 1,470 mN (i.e., atension of 150 gf on a 300-mm diameter quartz cylinder). As comparedwith the test conditions of Example 5, the test conditions of Example 6employed finer grade sandpaper (i.e., 220-grit rather than 180-grit).

As in Example 3, using matched fiber samples, fiber attenuation wasmeasured after winding at room temperature (i.e., 23° C.). Then, thedrum (with about 400 meters of wound fiber) was temperature cycled fromabout room temperature through (i) −40° C., (ii) −60° C., and (iii)+23°C. (i.e., near room temperature) while making loss measurements at 1550nanometers using an optical time domain reflectometer (OTDR).

Three (3) samples of each kind of optical fiber were initially measuredat 23° C. on the original spools (i.e., before winding on the rougheneddrum surface to establish baseline spectral attenuation) and then weresubjected to the foregoing rigorous testing conditions for one hour ateach temperature. Fiber attenuation was measured after one hour at eachtemperature.

FIG. 10 depicts exemplary results for single-mode optical fibers thatinclude a conventional primary coating (i.e., DeSolite® DP1007UV-curable urethane acrylate) and for bend-insensitive glass fibers(e.g., Draka Comteq's single-mode glass fibers available under the tradename BendBright^(XS)®) that include a primary coating having very lowmodulus (i.e., DSM Desotech's UV-curable urethane acrylate productprovided under the trade name DeSolite® DP 1011).

FIG. 10 demonstrates that the optical fiber according to the presentinvention, namely Draka Comteq's single-mode glass fibers availableunder the trade name BendBright^(XS)® with a primary coating having verylow modulus (e.g., DSM Desotech's UV-curable urethane acrylate productprovided under the trade name DeSolite® DP 1011), provides exceptionallylow attenuation losses as compared with standard single-mode opticalfibers (SSMF).

In addition, FIGS. 11 and 12 depict attenuation and microbendsensitivity, respectively, at a wavelength of 1550 nanometers as afunction of MAC number (i.e., mode field diameter divided by cutoffwavelength) for various exemplary optical fibers in accordance with thestandard IEC fixed-diameter sandpaper drum test (i.e., IEC TR62221,Method B). The respective attenuation data depicted in FIG. 11 (addedloss) and FIG. 12 (microbend sensitivity) were obtained at 23° C. underthe test conditions previously described with respect to FIG. 1 (i.e.,400-meter fiber samples were wound at about 2,940 mN (i.e., a tension of300 gf) on a 300-mm diameter fiber spool wrapped with adhesive-backed,40-micron grade sandpaper).

FIG. 11 shows that Draka Comteq's bend-resistant, single-mode glassfiber available under the trade name BendBright^(XS)® in combinationwith Draka Comteq's ColorLock^(XS) brand coating system providesoutstanding performance with respect to added loss.

FIG. 12 shows that Draka Comteq's bend-resistant, single-mode glassfiber available under the trade name BendBright^(XS)® in combinationwith Draka Comteq's ColorLock^(XS) brand coating system providessuperior microbend sensitivity (i.e., microbend sensitivity of 0.01 to0.03 (dB/km)/(gf/mm)).

The optical fibers according to the present invention typically furtherinclude a tough secondary coating to protect the primary coating andglass fiber from damage during handling and installation. For example,the secondary coating might have a modulus of between about 800 and1,000 MPa (e.g., about 900 MPa) as measured on a standard 75-micronfilm. As disclosed herein, this secondary coating may be inked as acolor code or, preferably, may be color-inclusive to provideidentification without the need for a separate inking process.

In one embodiment according to the present invention, the secondarycoating, which surrounds the primary coating to thereby protect thefiber structure, features an inclusive coloring system (i.e., notrequiring an extra layer of ink to be added for color coding). Thecolors, which conform to Munsell standards for optical fibercolor-coding, are enhanced for brightness and visibility under dimlighting (e.g., in deep shade or in confined spaces, such as manholes)and are easily distinguished against both light and dark backgrounds.

Furthermore, the secondary coating features a surface that provides anexcellent interface with ribbon matrix material so that the matrixseparates easily from the colored fiber in a way that does not sacrificerobustness. The mechanical properties of the colored secondary coatingare balanced with those of the primary coating so that, in heatstripping, the coating/matrix composite separates cleanly from the glassfibers.

Employing Draka Comteq's bend-resistant, single-mode glass fiberavailable under the trade name BendBright^(XS®) with the presentdual-coating system, which includes a low-modulus primary coating, hasbeen found to reduce microbending sensitivity by between about one totwo orders of magnitude relative to standard single-mode fiber (SSMF) atthe key transmission frequencies of 1550 nanometers and 1625 nanometers.As noted, such optical fiber not only provides outstanding resistance tomicrobending and macrobending, but also complies with the ITU-TG.657.A/B and ITU-T G.652.D requirements. Table 2(below) depictsoptical-fiber attributes of an exemplary bend-insensitive optical fiberin accordance with the present invention.

TABLE 2 (Exemplary Optical-Fiber Attributes) Attribute Detail Value ModeField Wavelength (nm) 1310 Diameter Range of Nominal 8.5-9.3 Values (μm)Cladding Diameter Nominal (μm) 125 Tolerance (μm) ±0.7 CoreConcentricity Maximum (μm) 0.5 Error Cladding Maximum (%) 0.7Non-Circularity Cable Cut-Off Maximum (nm) 1260 Wavelength MacrobendingRadius (mm) 15 10 7.5 Loss Number of Turns 10 1 1 Maximum @ 1550 nm (dB)0.03 0.1 0.5 Maximum @ 1625 nm (dB) 0.1 0.2 1.0 Proof Stress Minimum(GPa) 0.7 Chromatic λ_(0 min) (nm) 1300 Dispersion λ_(0 max) (nm) 1324Coefficient S_(0 max) (ps/(nm² · km)) ≦0.092

In particular, Draka Comteq's bend-resistant, single-mode glass fiberavailable under the trade name BendBright^(XS)® (e.g., enhanced withDraka Comteq's ColorLock^(XS) brand coating system) provides resistanceto macrobending required for sustained bends having a radius as low asfive (5) millimeters with an estimated failure probability of less thantwo (2) breaks per million full circle bends (i.e., 360°) over 30 yearsin a properly protected environment. These bend-resistant optical fibersfacilitate the rapid deployment of small, flexible cables for thedelivery of fiber to the premises/business/home (i.e., FTTx) by virtueof the optical fiber's ability to sustain a loss-free transmissionthrough small-radius bends. Cables employing such bend-resistant opticalfibers may be routed around sharp bends, stapled to building frame,coiled, and otherwise employed in demanding environments while retainingclear and strong signal transmission.

* * *

The bend-insensitive optical fibers according to the present inventionfacilitate the reduction in overall optical-fiber diameter. As will beappreciated by those having ordinary skill in the art, areduced-diameter optical fiber is cost-effective, requiring less rawmaterial. Moreover, a reduced-diameter optical fiber requires lessdeployment space (e.g., within a buffer tube and/or fiber optic cable),thereby facilitating increased fiber count and/or reduced cable size.

Those having ordinary skill in the art will recognize that an opticalfiber with a primary coating (and an optional secondary coating and/orink layer) typically has an outer diameter of between about 235 micronsand about 265 microns (μm). The component glass fiber itself (i.e., theglass core and surrounding cladding layers) typically has a diameter ofabout 125 microns, such that the total coating thickness is typicallybetween about 55 microns and 70 microns.

With respect to the optical fiber according to the present invention,the component glass fiber typically has an outer diameter of about 125microns. With respect to the optical fiber's surrounding coating layers,the primary coating typically has an outer diameter of between about 175microns and about 195 microns (i.e., a primary coating thickness ofbetween about 25 microns and 35 microns) and the secondary coatingtypically has an outer diameter of between about 235 microns and about265 microns (i.e., a secondary coating thickness of between about 20microns and 45 microns). Optionally, the optical fiber according to thepresent invention may include an outermost ink layer, which is typicallybetween two and ten microns.

In an alternative embodiment, an optical fiber according to the presentinvention may possess a reduced diameter (e.g., an outermost diameterbetween about 150 microns and 230 microns). In this alternative opticalfiber configuration, the thickness of the primary coating and/orsecondary coating is reduced, while the diameter of the component glassfiber is maintained at about 125 microns. By way of example, in suchembodiments the primary coating layer may have an outer diameter ofbetween about 135 microns and about 175 microns (e.g., about 160microns), and the secondary coating layer may have an outer diameter ofbetween about 150 microns and about 230 microns (e.g., more than about165 microns, such as 190-210 microns or so). In other words, the totaldiameter of the optical fiber is reduced to less than about 230 microns(e.g., about 200 microns).

* * *

As noted, the optical fiber according to the present invention includesone or more coating layers (e.g., a primary coating and a secondarycoating). At least one of the coating layers—typically the secondarycoating—may be colored and/or possess other markings to help identifyindividual fibers. Alternatively, a tertiary ink layer may surround theprimary and secondary coatings.

As discussed previously, combining (i) a coating system according to thepresent invention with (ii) a glass fiber having a refractive indexprofile that itself provides bend resistance (e.g., low macrobendingsensitivity) has been found to provide unexpectedly superior reductionsin microbend sensitivity. Indeed, bend-insensitive glass fibers areespecially suitable for use with the coating system of the presentinvention (e.g., Draka Comteq's ColorLock^(XS) brand coating system).

The optical fiber according to the present invention may be deployed invarious structures, such as those exemplary structures disclosedhereinafter.

For example, one or more of the present optical fibers may be enclosedwithin a buffer tube. For instance, optical fiber may be deployed ineither a single fiber loose buffer tube or a multi-fiber loose buffertube. With respect to the latter, multiple optical fibers may be bundledor stranded within a buffer tube or other structure. In this regard,within a multi-fiber loose buffer tube, fiber sub-bundles may beseparated with binders (e.g., each fiber sub-bundle is enveloped in abinder). Moreover, fan-out tubing may be installed at the termination ofsuch loose buffer tubes to directly terminate loose buffered opticalfibers with field-installed connectors.

In other embodiments, the buffer tube may tightly surround the outermostoptical fiber coating (i.e., tight buffered fiber) or otherwise surroundthe outermost optical fiber coating or ink layer to provide an exemplaryradial clearance of between about 50 and 100 microns (i.e., a semi-tightbuffered fiber).

With respect to the former tight buffered fiber, the buffering may beformed by coating the optical fiber with a curable composition (e.g., aUV-curable material) or a thermoplastic material. The outer diameter oftight buffer tubes, regardless of whether the buffer tube is formed froma curable or non-curable material, is typically less about 1,000 microns(e.g., either about 500 microns or about 900 microns).

With respect to the latter semi-tight buffered fiber, a lubricant may beincluded between the optical fiber and the buffer tube (e.g., to providea gliding layer).

As will be known by those having ordinary skill in the art, an exemplarybuffer tube enclosing optical fibers as disclosed herein may be formedof polyolefins (e.g., polyethylene or polypropylene), includingfluorinated polyolefins, polyesters (e.g., polybutylene terephthalate),polyamides (e.g., nylon), as well as other polymeric materials andblends. In general, a buffer tube may be formed of one or more layers.The layers may be homogeneous or include mixtures or blends of variousmaterials within each layer.

In this context, the buffer tube may be extruded (e.g., an extrudedpolymeric material) or pultruded (e.g., a pultruded, fiber-reinforcedplastic). By way of example, the buffer tube may include a material toprovide high temperature and chemical resistance (e.g., an aromaticmaterial or polysulfone material).

Although buffer tubes typically have a circular cross section, buffertubes alternatively may have an irregular or non-circular shape (e.g.,an oval or a trapezoidal cross-section).

Alternatively, one or more of the present optical fibers may simply besurrounded by an outer protective sheath or encapsulated within a sealedmetal tube. In either structure, no intermediate buffer tube isnecessarily required.

Multiple optical fibers as disclosed herein may be sandwiched,encapsulated, and/or edge bonded to form an optical fiber ribbon.Optical fiber ribbons can be divisible into subunits (e.g., atwelve-fiber ribbon that is splittable into six-fiber subunits).Moreover, a plurality of such optical fiber ribbons may be aggregated toform a ribbon stack, which can have various sizes and shapes.

For example, it is possible to form a rectangular ribbon stack or aribbon stack in which the uppermost and lowermost optical fiber ribbonshave fewer optical fibers than those toward the center of the stack.This construction may be useful to increase the density of opticalelements (e.g., optical fibers) within the buffer tube and/or cable.

In general, it is desirable to increase the filling of transmissionelements in buffer tubes or cables, subject to other constraints (e.g.,cable or mid-span attenuation). The optical elements themselves may bedesigned for increased packing density. For example, the optical fibermay possess modified properties, such as improved refractive-indexprofile, core or cladding dimensions, or primary coating thicknessand/or modulus, to improve microbending and macrobendingcharacteristics.

By way of example, a rectangular ribbon stack may be formed with orwithout a central twist (i.e., a “primary twist”). Those having ordinaryskill in the art will appreciate that a ribbon stack is typicallymanufactured with rotational twist to allow the tube or cable to bendwithout placing excessive mechanical stress on the optical fibers duringwinding, installation, and use. In a structural variation, a twisted (oruntwisted) rectangular ribbon stack may be further formed into acoil-like configuration (e.g., a helix) or a wave-like configuration(e.g., a sinusoid). In other words, the ribbon stack may possess regular“secondary” deformations.

As will be known to those having ordinary skill in the art, such opticalfiber ribbons may be positioned within a buffer tube or othersurrounding structure, such as a buffer-tube-free cable. Subject tocertain restraints (e.g., attenuation) it is desirable to increase thedensity of elements such as optical fibers or optical fiber ribbonswithin buffer tubes and/or optical fiber cables.

A plurality of buffer tubes containing optical fibers (e.g., loose orribbonized fibers) may be positioned externally adjacent to and strandedaround a central strength member. This stranding can be accomplished inone direction, helically, known as “S” or “Z” stranding, or ReverseOscillated Lay stranding, known as “S-Z” stranding. Stranding about thecentral strength member reduces optical fiber strain when cable strainoccurs during installation and use.

Those having ordinary skill in the art will understand the benefit ofminimizing fiber strain for both tensile cable strain and longitudinalcompressive cable strain during installation or operating conditions.

With respect to tensile cable strain, which may occur duringinstallation, the cable will become longer while the optical fibers canmigrate closer to the cable's neutral axis to reduce, if not eliminate,the strain being translated to the optical fibers. With respect tolongitudinal compressive strain, which may occur at low operatingtemperatures due to shrinkage of the cable components, the opticalfibers will migrate farther away from the cable's neutral axis toreduce, if not eliminate, the compressive strain being translated to theoptical fibers.

In a variation, two or more substantially concentric layers of buffertubes may be positioned around a central strength member. In a furthervariation, multiple stranding elements (e.g., multiple buffer tubesstranded around a strength member) may themselves be stranded aroundeach other or around a primary central strength member.

Alternatively, a plurality of buffer tubes containing optical fibers(e.g., loose or ribbonized fibers) may be simply placed externallyadjacent to the central strength member (i.e., the buffer tubes are notintentionally stranded or arranged around the central strength member ina particular manner and run substantially parallel to the centralstrength member).

Alternatively still, the present optical fibers may be positioned with acentral buffer tube (i.e., the central buffer tube cable has a centralbuffer tube rather than a central strength member). Such a centralbuffer tube cable may position strength members elsewhere. For instance,metallic or non-metallic (e.g., GRP) strength members may be positionedwithin the cable sheath itself, and/or one or more layers ofhigh-strength yarns (e.g., aramid or non-aramid yarns) may be positionedparallel to or wrapped (e.g., contrahelically) around the central buffertube (i.e., within the cable's interior space). Likewise, strengthmembers can be included within the buffer tube's casing.

In other embodiments, the optical fibers may be placed within a slottedcore cable. In a slotted core cable, optical fibers, individually or asa fiber ribbon, may be placed within pre-shaped helical grooves (i.e.,channels) on the surface of a central strength member, thereby forming aslotted core unit. The slotted core unit may be enclosed by a buffertube. One or more of such slotted core units may be placed within aslotted core cable. For example, a plurality of slotted core units maybe helically stranded around a central strength member.

Alternatively, the optical fibers may also be stranded in a maxitubecable design, whereby the optical fibers are stranded around themselveswithin a large multi-fiber loose buffer tube rather than around acentral strength member. In other words, the large multi-fiber loosebuffer tube is centrally positioned within the maxitube cable. Forexample, such maxitube cables may be deployed in optical ground wires(OPGW).

In another cabling embodiment, multiple buffer tubes may be strandedaround themselves without the presence of a central member. Thesestranded buffer tubes may be surrounded by a protective tube. Theprotective tube may serve as the outer casing of the fiber optic cableor may be further surrounded by an outer sheath. The protective tube maytightly or loosely surround the stranded buffer tubes.

As will be known to those having ordinary skill in the art, additionalelements may be included within a cable core. For example, copper cablesor other active, transmission elements may be stranded or otherwisebundled within the cable sheath. Passive elements may also be placedwithin the cable core, such as between the interior walls of the buffertubes and the enclosed optical fibers. Alternatively and by way ofexample, passive elements may be placed outside the buffer tubes betweenthe respective exterior walls of the buffer tubes and the interior wallof the cable jacket, or, within the interior space of a buffer-tube-freecable.

For example, yarns, nonwovens, fabrics (e.g., tapes), foams, or othermaterials containing water-swellable material and/or coated withwater-swellable materials (e.g., including super absorbent polymers(SAPs), such as SAP powder) may be employed to provide water blockingand/or to couple the optical fibers to the surrounding buffer tubeand/or cable jacketing (e.g., via adhesion, friction, and/orcompression). Exemplary water-swellable elements are disclosed incommonly assigned U.S. Patent Application Publication No. US2007/0019915 A1 and its related U.S. patent application Ser. No.11/424,112 for a Water-Swellable Tape, Adhesive-Backed for Coupling WhenUsed Inside a Buffer Tube (Overton et al.), each of which is herebyincorporated by reference in its entirety.

Moreover, an adhesive (e.g., a hot-melt adhesive or curable adhesive,such as a silicone acrylate cross-linked by exposure to actinicradiation) may be provided on one or more passive elements (e.g.,water-swellable material) to bond the elements to the buffer tube. Anadhesive material may also be used to bond the water-swellable elementto optical fibers within the buffer tube. Exemplary arrangements of suchelements are disclosed in commonly assigned U.S. Patent ApplicationPublication No. US 2008/0145010 A1 for a Gel-Free Buffer Tube withAdhesively Coupled Optical Element (Overton et al.), which is herebyincorporated by reference in its entirety.

The buffer tubes (or buffer-tube-free cables) may also contain athixotropic composition (e.g., grease or grease-like gels) between theoptical fibers and the interior walls of the buffer tubes. For example,filling the free space inside a buffer tube with water-blocking,petroleum-based filling grease helps to block the ingress of water.Further, the thixotropic filling grease mechanically (i.e., viscously)couples the optical fibers to the surrounding buffer tube.

Such thixotropic filling greases are relatively heavy and messy, therebyhindering connection and splicing operations. Thus, the present opticalfibers may be deployed in dry cable structures (i.e., grease-free buffertubes).

Exemplary buffer tube structures that are free from thixotropic fillinggreases are disclosed in commonly assigned U.S. patent application Ser.No. 12/146,588 for a Coupling Composition for Optical Fiber Cables,filed Jun. 26, 2008, (Parris et al.), which is hereby incorporated byreference in its entirety. Such buffer tubes employ couplingcompositions formed from a blend of high-molecular weight elastomericpolymers (e.g., about 35 weight percent or less) and oils (e.g., about65 weight percent or more) that flow at low temperatures. Unlikethixotropic filling greases, the coupling composition (e.g., employed asa cohesive gel or foam) is typically dry and, therefore, less messyduring splicing.

As will be understood by those having ordinary skill in the art, a cableenclosing optical fibers as disclosed herein may have a sheath formedfrom various materials in various designs. Cable sheathing may be formedfrom polymeric materials such as, for example, polyethylene,polypropylene, polyvinyl chloride (PVC), polyamides (e.g., nylon),polyester (e.g., PBT), fluorinated plastics (e.g., perfluorethylenepropylene, polyvinyl fluoride, or polyvinylidene difluoride), andethylene vinyl acetate. The sheath and/or buffer tube materials may alsocontain other additives, such as nucleating agents, flame-retardants,smoke-retardants, antioxidants, UV absorbers, and/or plasticizers.

The cable sheathing may be a single jacket formed from a dielectricmaterial (e.g., non-conducting polymers), with or without supplementalstructural components that may be used to improve the protection (e.g.,from rodents) and strength provided by the cable sheath. For example,one or more layers of metallic (e.g., steel) tape along with one or moredielectric jackets may form the cable sheathing. Metallic or fiberglassreinforcing rods (e.g., GRP) may also be incorporated into the sheath.In addition, aramid, fiberglass, or polyester yarns may be employedunder the various sheath materials (e.g., between the cable sheath andthe cable core), and/or ripcords may be positioned, for example, withinthe cable sheath.

Similar to buffer tubes, optical fiber cable sheaths typically have acircular cross section, but cable sheaths alternatively may have anirregular or non-circular shape (e.g., an oval, trapezoidal, or flatcross-section).

By way of example, the optical fiber according to the present inventionmay be incorporated into single-fiber drop cables, such as thoseemployed for Multiple Dwelling Unit (MDU) applications. In suchdeployments, the cable jacketing must exhibit crush resistance, abrasionresistance, puncture resistance, thermal stability, and fire resistanceas required by building codes. An exemplary material for such cablejackets is thermally stable, flame-retardant polyurethane (PUR), whichmechanically protects the optical fibers yet is sufficiently flexible tofacilitate easy MDU installations. Alternatively, a flame-retardantpolyolefin or polyvinyl chloride sheath may be used.

In general and as will be known to those having ordinary skill in theart, a strength member is typically in the form of a rod orbraided/helically wound wires or fibers, though other configurationswill be within the knowledge of those having ordinary skill in the art.

Optical fiber cables containing optical fibers as disclosed may bevariously deployed, including as drop cables, distribution cables,feeder cables, trunk cables, and stub cables, each of which may havevarying operational requirements (e.g., temperature range, crushresistance, UV resistance, and minimum bend radius).

Such optical fiber cables may be installed within ducts, microducts,plenums, or risers. By way of example, an optical fiber cable may beinstalled in an existing duct or microduct by pulling or blowing (e.g.,using compressed air). An exemplary cable installation method isdisclosed in commonly assigned U.S. Patent Application Publication No.2007/0263960 for a Communication Cable Assembly and Installation Method(Lock et al.), and U.S. patent application Ser. No. 12/200,095 for aModified Pre-Ferrulized Communication Cable Assembly and InstallationMethod, filed Aug. 28, 2008, (Griffioen et al.), each of which isincorporated by reference in its entirety.

As noted, buffer tubes containing optical fibers (e.g., loose orribbonized fibers) may be stranded (e.g., around a central strengthmember). In such configurations, an optical fiber cable's protectiveouter sheath may have a textured outer surface that periodically varieslengthwise along the cable in a manner that replicates the strandedshape of the underlying buffer tubes. The textured profile of theprotective outer sheath can improve the blowing performance of theoptical fiber cable. The textured surface reduces the contact surfacebetween the cable and the duct or microduct and increases the frictionbetween the blowing medium (e.g., air) and the cable. The protectiveouter sheath may be made of a low coefficient-of-friction material,which can facilitate blown installation. Moreover, the protective outersheath can be provided with a lubricant to further facilitate blowninstallation.

In general, to achieve satisfactory long-distance blowing performance(e.g., between about 3,000 to 5,000 feet or more), the outer cablediameter of an optical fiber cable should be no more than about seventyto eighty percent of the duct's or microducts inner diameter.

Compressed air may also be used to install optical fibers according tothe present invention in an air blown fiber system. In an air blownfiber system, a network of unfilled cables or microducts is installedprior to the installation of optical fibers. Optical fibers maysubsequently be blown into the installed cables as necessary to supportthe network's varying requirements.

Moreover, the optical fiber cables may be directly buried in the groundor, as an aerial cable, suspended from a pole or pylon. An aerial cablemay be self-supporting or secured or lashed to a support (e.g.,messenger wire or another cable). Exemplary aerial fiber optic cablesinclude overhead ground wires (OPGW), all-dielectric self-supportingcables (ADSS), all dielectric lash cables (AD-Lash), and figure-eightcables, each of which is well understood by those having ordinary skillin the art. (Figure-eight cables and other designs can be directlyburied or installed into ducts, and may optionally include a toningelement, such as a metallic wire, so that they can be found with a metaldetector.

In addition, although the optical fibers may be further protected by anouter cable sheath, the optical fiber itself may be further reinforcedso that the optical fiber may be included within a breakout cable, whichallows for the individual routing of individual optical fibers.

To effectively employ the present optical fibers in a transmissionsystem, connections are required at various points in the network.Optical fiber connections are typically made by fusion splicing,mechanical splicing, or mechanical connectors.

The mating ends of connectors can be installed to the fiber ends eitherin the field (e.g., at the network location) or in a factory prior toinstallation into the network. The ends of the connectors are mated inthe field in order to connect the fibers together or connect the fibersto the passive or active components. For example, certain optical fibercable assemblies (e.g., furcation assemblies) can separate and conveyindividual optical fibers from a multiple optical fiber cable toconnectors in a protective manner.

The deployment of such optical fiber cables may include supplementalequipment. For instance, an amplifier may be included to improve opticalsignals. Dispersion compensating modules may be installed to reduce theeffects of chromatic dispersion and polarization mode dispersion. Spliceboxes, pedestals, and distribution frames, which may be protected by anenclosure, may likewise be included. Additional elements include, forexample, remote terminal switches, optical network units, opticalsplitters, and central office switches.

A cable containing optical fibers according to the present invention maybe deployed for use in a communication system (e.g., networking ortelecommunications). A communication system may include fiber opticcable architecture such as fiber-to-the-node (FTTN),fiber-to-the-telecommunications enclosure (FTTE), fiber-to-the-curb(FTTC), fiber-to-the-building (FTTB), and fiber-to-the-home (FTTH), aswell as long-haul or metro architecture. Moreover, an optical module ora storage box that includes a housing may receive a wound portion of theoptical fiber disclosed herein. By way of example, the optical fiber maybe wound with a bending radius of less than about 15 millimeters (e.g.,10 millimeters or less, such as about 5 millimeters) in the opticalmodule or the storage box.

Moreover, optical fibers according to the present invention may be usedin other applications, including, without limitation, fiber opticsensors or illumination applications (e.g., lighting).

In another aspect, the optical fibers according to the present inventionmay be enclosed by buffer tubes formed from a hardened polymer material(e.g., polysulfone).

Those having ordinary skill in the art will appreciate that hardenedbuffer tubes subject conventional optical fibers to excessive risk ofmicrobending. In contrast and as noted, the present bend-insensitiveoptical fibers provide exceptional microbending resistance, and so canbe satisfactory deployed in hardened buffer tubes.

By way of example, the hardened buffer tube may have an outer diameterbetween about one and two millimeters. An exemplary hardened buffer tubemay possess an inner diameter of about 300 microns, thus forming asingle-fiber, semi-tight buffer tube (e.g., a hardened buffer tubehaving an outer diameter of 1.0 millimeters and an inner diameter ofabout 300 microns).

In a particular embodiment, a bend-insensitive optical fiber accordingto the present invention may be enclosed by a hardened buffer tubeformed from polysulfone, such as by extrusion or pultrusion. This kindof hardened buffer tube provides superior resistance to lateral stressesthat could otherwise cause microbending or macrobending of the enclosedoptical fiber. The hardened buffer tube is capable of withstanding hightemperatures (e.g., 200° C.) and exposure to corrosive chemicals (e.g.,gasoline). Similar to more complex structures, the present hardenedbuffer tube offers protection against lateral stresses, hightemperatures, and corrosive chemicals yet is less expensive and simplerto manufacture.

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This application incorporates entirely by reference the followingcommonly assigned patent, patent application publications, and patentapplications: U.S. Pat. No. 5,574,816 for Polypropylene-PolyethyleneCopolymer Buffer Tubes for Optical Fiber Cables and Method for Makingthe Same, and its related patent application Ser. No. 08/377,366; U.S.Pat. No. 5,717,805 for Stress Concentrations in an Optical Fiber Ribbonto Facilitate Separation of Ribbon Matrix Material; U.S. Pat. No.5,761,362 for Polypropylene-Polyethylene Copolymer Buffer Tubes forOptical Fiber Cables and Method for Making the Same; U.S. Pat. No.5,911,023 for Polyolefin Materials Suitable for Optical Fiber CableComponents; U.S. Pat. No. 5,982,968 for Stress Concentrations in anOptical Fiber Ribbon to Facilitate Separation of Ribbon Matrix Material;U.S. Pat. No. 6,035,087 for Optical Unit for Fiber Optic Cables; U.S.Pat. No. 6,066,397 for Polypropylene Filler Rods for Optical FiberCommunications Cables; U.S. Pat. No. 6,175,677 for Optical FiberMulti-Ribbon and Method for Making the Same; U.S. Pat. No. 6,085,009 forWater Blocking Gels Compatible with Polyolefin Optical Fiber CableBuffer Tubes and Cables Made Therewith; U.S. Pat. No. 6,215,931 forFlexible Thermoplastic Polyolefin Elastomers for Buffering TransmissionElements in a Telecommunications Cable; U.S. Pat. No. 6,134,363 forMethod for Accessing Optical Fibers in the Midspan Region of an OpticalFiber Cable; U.S. Pat. No. 6,381,390 for Color-Coded Optical FiberRibbon and Die for Making the Same; U.S. Pat. No. 6,181,857 for Methodfor Accessing Optical Fibers Contained in a Sheath; U.S. Pat. No.6,314,224 for Thick-Walled Cable Jacket with Non-Circular Cavity CrossSection; U.S. Pat. No. 6,334,016 for Optical Fiber Ribbon MatrixMaterial Having Optimal Handling Characteristics; U.S. Pat. No.6,321,012 for Optical Fiber Having Water Swellable Material forIdentifying Grouping of Fiber Groups; U.S. Pat. No. 6,321,014 for Methodfor Manufacturing Optical Fiber Ribbon; U.S. Pat. No. 6,210,802 forPolypropylene Filler Rods for Optical Fiber Communications Cables; U.S.Pat. No. 6,493,491 for Optical Drop Cable for Aerial Installation; U.S.Pat. No. 7,346,244 for Coated Central Strength Member for Fiber OpticCables with Reduced Shrinkage; U.S. Pat. No. 6,658,184 for ProtectiveSkin for Optical Fibers; U.S. Pat. No. 6,603,908 for Buffer Tube thatResults in Easy Access to and Low Attenuation of Fibers Disposed WithinBuffer Tube; U.S. Pat. No. 7,045,010 for Applicator for High-Speed GelBuffering of Flextube Optical Fiber Bundles; U.S. Pat. No. 6,749,446 forOptical Fiber Cable with Cushion Members Protecting Optical Fiber RibbonStack; U.S. Pat. No. 6,922,515 for Method and Apparatus to ReduceVariation of Excess Fiber Length in Buffer Tubes of Fiber Optic Cables;U.S. Pat. No. 6,618,538 for Method and Apparatus to Reduce Variation ofExcess Fiber Length in Buffer Tubes of Fiber Optic Cables; U.S. Pat. No.7,322,122 for Method and Apparatus for Curing a Fiber Having at LeastTwo Fiber Coating Curing Stages; U.S. Pat. No. 6,912,347 for OptimizedFiber Optic Cable Suitable for Microduct Blown Installation; U.S. PatentNo. 6,941,049 for Fiber Optic Cable Having No Rigid Strength Members anda Reduced Coefficient of Thermal Expansion; U.S. Pat. No. 7,162,128 forUse of Buffer Tube Coupling Coil to Prevent Fiber Retraction; U.S.Patent Application Publication No. US 2007/0019915 A1 for aWater-Swellable Tape, Adhesive-Backed for Coupling When Used Inside aBuffer Tube (Overton et al.); International Patent ApplicationPublication No. 2007/013923 for Grease-Free Buffer Optical Fiber BufferTube Construction Utilizing a Water-Swellable, Texturized Yarn (Overtonet al.); European Patent Application Publication No. 1,921,478 A1, for aTelecommunication Optical Fiber Cable (Tatat et al.); U.S. PatentApplication Publication No. US 2007/0183726 A1 for a Optical Fiber CableSuited for Blown Installation or Pushing Installation in Microducts ofSmall Diameter (Nothofer et al.); U.S. Patent Application PublicationNo. US 2008/0037942 A1 for an Optical Fiber Telecommunications Cable(Tatat); U.S. Patent Application Publication No. US 2008/0145010 A1 fora Gel-Free Buffer Tube with Adhesively Coupled Optical Element (Overtonet al.); U.S. Patent Application Publication No. US 2008/0181564 A1 fora Fiber Optic Cable Having a Water-Swellable Element (Overton); U.S.patent application Ser. No. 12/101/528 for a Method for AccessingOptical Fibers within a Telecommunication Cable, filed Apr. 11, 2008,(Lavenne et al.); U.S. patent application Ser. No. 12/146,526 for aOptical Fiber Cable Having a Deform able Coupling Element, filed Jun.26, 2008, (Parris et al.); U.S. patent application Ser. No. 12/146,535for an Optical Fiber Cable Having Raised Coupling Supports, filed Jun.26, 2008, (Parris); U.S. patent application Ser. No. 12/146,588 for aCoupling Composition for Optical Fiber Cables, filed Jun. 26, 2008,(Parris et al.); U.S. Patent Application No. 61/096,545 for a OpticalFiber Cable Assembly, filed Sep. 12, 2008, (Barker et al.); U.S. PatentApplication No. 61/096,750 for a High-Fiber-Density Optical Fiber Cable,filed Sep. 12, 2008, (Lovie, et al.).

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In the specification and figures, typical embodiments of the inventionhave been disclosed. The present invention is not limited to suchexemplary embodiments. Unless otherwise noted, specific terms have beenused in a generic and descriptive sense and not for purposes oflimitation.

1. An optical fiber possessing an improved coating system that reducesstress-induced microbending, the optical fiber comprising: a single-modeglass fiber; and a substantially cured primary coating surrounding saidglass fiber, said substantially cured primary coating possessing (i) anin situ modulus of less than 0.50 MPa and (ii) a glass transitiontemperature of less than −55° C.; wherein, at a wavelength of 1310nanometers, the optical fiber has a mode field diameter with nominalvalues of between 8.5 microns and 9.3 microns; wherein, the opticalfiber has a standard cable cut-off wavelength of no more than 1260nanometers; wherein, the optical fiber has a zero chromatic dispersionwavelength of at least 1300 nanometers and no more than 1324 nanometers;wherein, at the zero chromatic dispersion wavelength, the optical fiberhas a slope of no more than 0.092 ps/(nm²·km); wherein, at a wavelengthof 1550 nanometers, the optical fiber has induced bending attenuation of(i) 0.03 dB or less for ten turns around a mandrel radius of 15millimeters, (ii) 0.1 dB or less for one turn around a mandrel radius of10 millimeters, and (iii) 0.5 dB or less for one turn around a mandrelradius of 7.5 millimeters; and wherein, at a wavelength of 1625nanometers, the optical fiber has induced bending attenuation of (i) 0.1dB or less for ten turns around a mandrel radius of 15 millimeters, (ii)0.2 dB or less for one turn around a mandrel radius of 10 millimeters,and (iii) 1.0 dB or less for one turn around a mandrel radius of 7.5millimeters.
 2. An optical fiber according to claim 1, wherein saidsubstantially cured primary coating possesses an in situ modulus of morethan about 0.2 MPa.
 3. An optical fiber according to claim 1, whereinsaid substantially cured primary coating possesses an in situ modulus ofbetween about 0.3 MPa and 0.4 MPa.
 4. An optical fiber according toclaim 1, wherein said substantially cured primary coating possesses aglass transition temperature of less than −60° C.
 5. An optical fiberaccording to claim 1, wherein said primary coating achieves 50 percentof full cure at a UV dose of about 0.3 J/cm² as measured on a standard75-micron film at standard temperature and pressure.
 6. An optical fiberaccording to claim 1, wherein said primary coating achieves 90 percentof full cure at a UV dose of about 1.0 J/cm² as measured on a standard75-micron film at standard temperature and pressure.
 7. An optical fiberaccording to claim 1, wherein said primary coating comprises aUV-curable urethane acrylate composition.
 8. A buffer tube or cablecontaining one or more optical fibers according to claim
 1. 9. An FTTxinstallation comprising one or more optical fibers according to claim 1.10. An optical fiber, comprising: a trench-assisted, single-mode glassfiber having an outer diameter of about 125 microns; a substantiallycured primary coating surrounding said glass fiber, said substantiallycured primary coating having a thickness of between about 25 microns and35 microns and possessing (i) an in situ modulus of between about 0.2MPa and 0.40 MPa and (ii) a glass transition temperature of less than−55° C.; and a secondary coating surrounding said primary coating, saidsecondary coating having a thickness of between about 20 microns and 45microns, wherein the in situ modulus of said secondary coating isgreater than the in situ modulus of said primary coating; wherein, at awavelength of 1310 nanometers, the optical fiber has a mode fielddiameter of between about 8.5 microns and 9.3 microns; wherein, theoptical fiber has a standard cable cut-off wavelength of 1260 nanometersor less; wherein, the optical fiber has a zero chromatic dispersionwavelength of between 1300 nanometers and 1324 nanometers; wherein, atthe zero chromatic dispersion wavelength, the optical fiber has a slopeof 0.092 ps/(nm²·km) or less; wherein, at a wavelength of 1550nanometers, the optical fiber has induced bending attenuation of (i)0.03 dB or less for ten turns around a mandrel radius of 15 millimeters,(ii) 0.1 dB or less for one turn around a mandrel radius of 10millimeters, and (iii) 0.5 dB or less for one turn around a mandrelradius of 7.5 millimeters; and wherein, at a wavelength of 1625nanometers, the optical fiber has induced bending attenuation of (i) 0.1dB or less for ten turns around a mandrel radius of 15 millimeters, (ii)0.2 dB or less for one turn around a mandrel radius of 10 millimeters,and (iii) 1.0 dB or less for one turn around a mandrel radius of 7.5millimeters.
 11. An optical fiber according to claim 10, wherein saidsubstantially cured primary coating possesses a glass transitiontemperature of less than −60° C.
 12. An optical fiber according to claim10, wherein said primary coating comprises between about 40 and 80weight percent of polyether-urethane acrylate oligomer.
 13. An opticalfiber according to claim 10, wherein, as measured on a standard75-micron film at standard temperature and pressure, said primarycoating achieves 50 percent of full cure at a UV dose of about 0.3J/cm², 80 percent of full cure at a UV dose of about 0.5 J/cm², and 90percent of full cure at a UV dose of about 1.0 J/cm².
 14. An opticalfiber according to claim 10, wherein, at wavelengths of 1550 nanometersand 1625 nanometers, the optical fiber possesses a spectral-attenuationadded loss of less than about 0.1 dB/km as measured in accordance withIEC TR62221, Method B (fixed diameter drum).
 15. An optical fiberaccording to claim 10, wherein, at a wavelength of 1550 nanometers, theoptical fiber possesses a spectral-attenuation added loss of less thanabout 0.05 dB/km as measured in accordance with IEC TR62221, Method B(fixed diameter drum).
 16. An optical fiber according to claim 10,wherein, at a wavelength of 1550 nanometers, the optical fiber possessesa spectral-attenuation added loss of less than 0.5 dB/km as measured at23° C. in accordance with a modified IEC TR62221 fixed-diametersandpaper drum test in which a 440-meter fiber sample is wound in asingle layer at about 1,470 mN on a 300-millimeter diameter quartz drumthat is wrapped with 180-grit sandpaper to create a rough surface. 17.An optical fiber according to claim 10, wherein, at a wavelength of 1550nanometers, the optical fiber possesses microbending sensitivity of (i)less than 75(dB/km)/(N/mm) at 23° C., (ii) less than 100 (dB/km)/(N/mm)at −40° C., and (iii) less than 200(dB/km)/(N/mm) at −60° C. as measuredin accordance with a modified IEC TR62221 fixed-diameter sandpaper drumtest in which a 440-meter fiber sample is wound in a single layer atabout 1,470 mN on a 300-millimeter diameter quartz drum that is wrappedwith 180-grit sandpaper to create a rough surface.
 18. An optical fiberaccording to claim 17, wherein, at a wavelength of 1550 nanometers, theoptical fiber possesses a spectral-attenuation added loss of less than0.75 dB/km as measured at −60° C. in accordance with a modified IECTR62221 fixed-diameter sandpaper drum test in which a 400-meter fibersample is wound in a single layer at about 1,470 mN on a 300-millimeterdiameter quartz drum that is wrapped with 220-grit sandpaper to create arough surface.
 19. An optical fiber according to claim 17, wherein, at awavelength of 1550 nanometers, the optical fiber possesses microbendingsensitivity of less than about 0.03(dB/km)/(gf/mm) as measured inaccordance with IEC TR62221, Method B (fixed diameter drum).
 20. Anoptical fiber according to claim 1, wherein, at a wavelength of 1550nanometers, the optical fiber possesses a spectral-attenuaton added lossof less than 0.2 dB/km as measured at −60° C. in accordance with amodified IEC TR62221, Method D (basketweave) test in which a fibersample is wound in fifty layers at about 490 mN on a 300-millimeterdiameter quartz drum with a 9-millimeter lay.
 21. An optical fiberaccording to claim 1, wherein, at a wavelength of 1550 nanometers, theoptical fiber possesses a spectral-attenuation added loss of less than0.7 dB/km as measured at −60° C. in accordance with a modified IECTR62221 fixed-diameter sandpaper drum test in which a 400-meter fibersample is wound in a single layer at about 1,470 mN on a 300-millimeterdiameter quartz drum that is wrapped with 220-grit sandpaper to create arough surface.
 22. An optical fiber according to claim 21, wherein: saidglass fiber has an outer diameter of about 125 microns; and the opticalfiber has an outer diameter of between about 235 microns and 265microns.
 23. An optical fiber according to claim 1, wherein, at awavelength of 1550 nanometers, the optical fiber possesses aspectral-attenuation added loss of less than 0.5 dB/km as measured at23° C. in accordance with a modified IEC TR62221 fixed-diametersandpaper drum test in which a 440-meter fiber sample is wound in asingle layer at about 1,470 mN on a 300-millimeter diameter quartz drumthat is wrapped with 180-grit sandpaper to create a rough surface. 24.An optical fiber according to claim 1, wherein, at a wavelength of 1550nanometers, the optical fiber possesses microbending sensitivity of (i)less than 75(dB/km)/(N/mm) at 23° C., (ii) less than 100(dB/km)/(N/mm)at −40° C., and (iii) less than 200(dB/km)/(N/mm) at −60° C. as measuredin accordance with a modified IEC TR62221 fixed-diameter sandpaper drumtest in which a 440-meter fiber sample is wound in a single layer atabout 1,470 mN on a 300-millimeter diameter quartz drum that is wrappedwith 180-grit sandpaper to create a rough surface.